The invention is directed to an oven used to anneal magnetic media for electronic applications and to methods of magnetic annealing employing the oven.
Magnetic vacuum ovens have been used in the manufacture of devices such as read/write heads for rigid media storage devices, e.g., magnetic resistive (MR) and giant magnetic resistive (GMR) heads, disk drives, xe2x80x9cMRAMxe2x80x9d wafers, and the like. Such magnetic media are referred to as wafers and typically are formed of a substrate bearing magnetic film or layers to which a particular magnetic orientation has been imparted through exposure to magnetic fields at elevated temperatures. The process of imparting a particular magnetic orientation in this manner is known as annealing or magnetic annealing.
In the annealing process, the magnetic media or wafers are heated to make them more susceptible to magnetic fields. The magnetic film or layers contain ferromagnetic material having a crystalline structure. Raising the temperature increases the vibrational moments of atoms forming the crystalline structure of the magnetic material and imparts a randomness to the motion of the atoms, weakening the crystalline structure of the ferromagnetic material in the magnetic film or layers. This places the atoms in a state that provides minimal resistance to the influence of an outside magnetic field. Exposing the heated wafers to a magnetic field causes the atoms to be held in place or oriented along the axis of the magnetic field. After subjecting the atoms or crystals of the magnetic media to elevated temperatures in the presence of a magnetic field of a desired strength for a prescribed period of time, the wafers are cooled, thus fixing or locking the atoms or crystals in the orientation imparted by the magnetic field. Thus, magnetic annealing involves both heating the media and subjecting the media to an magnetic field so as to orient the crystals of the magnetic film or layers thereof.
The magnetic annealing process may be carried out on a single wafer or on multiple wafers in batches or lots. The magnetic field can be generated by a permanent magnet, electromagnet or superconducting electromagnet. Such magnets have been incorporated into vacuum ovens. However, permanent magnets are heat sensitive, losing their magnetism at temperatures above their Curie point, and hence should not be positioned too closely to sources of heat. Though permanent magnets are utilized, electromagnets and superconducting electro-magnets are particularly well-suited for placement external to the chamber of a vacuum oven.
Despatch Industries, Inc. currently has commercially available annealing ovens, Model MT-300 and Model MT-500, that incorporate permanent magnets, electromagnets and superconducting electromagnets. Wafers are loaded onto individual shelves of a cassette that is placed into a carrier that may remain resident within the oven chamber at all times or may be removed from the chamber for loading and unloading. The carrier has a separate door which must be sealed prior to the beginning of the heating cycle. The entire carrier is made out of metal, but there is an elastomeric seal between the door and the rest of the carrier. The door of the carrier seals the carrier to prevent convective heat transfer between a heat transfer gas introduced into the chamber and the magnetic media within the carrier, and to minimize oxidation of the wafers. The chamber is filled with heated gas to raise the temperature of the carrier and the media therein to a desired annealing temperature through conductive heat transfer. A magnetic field is generated by magnets located outside of the oven chamber. The chamber is sealed with a door that typically also has an elastomeric seal. The desired temperatures for annealing typically are in excess of 300xc2x0 C. At these temperatures, the elastomeric seals tend to fail over time after a number of cycles. Consequently, the vacuum within the carrier cannot be properly maintained without necessary replacement of the elastomeric chamber door seal. In order to protect the magnetic media from exposure to convective heat and oxidation, replacement of the carrier door seal is also necessary.
It is therefore desirable to provide an magnetic annealing oven, that avoids the production losses resulting from seal failures and the process disruptions and downtime associated with seal replacement.
The present invention is directed to ovens for annealing magnetic media. The oven of the invention generally comprises a vacuum chamber formed of non-magnetic material having a heat exchanger disposed therein, and a magnet external to the chamber. The heat exchanger is connected to a heat transfer gas unit to form a closed heat transfer gas circuit. Magnetic media loaded in the heat exchanger are primarily heated and cooled by conductive heat transfer.
In an embodiment of the invention, the oven has a vacuum chamber formed of nonmagnetic material and a vacuum port with a vacuum seal through which magnetic media may be loaded and unloaded. Supported in the vacuum chamber is a heat exchanger which is also formed of non-magnetic material. The heat exchanger has a first compartment with an opening for receiving magnetic media to be treated in the oven and a second compartment in thermally conductive relationship with the first compartment. The second compartment has an airtight volume hermetically sealed from the first compartment and the vacuum chamber. The heat exchanger is spaced from the vacuum port so as to avoid heating of the vacuum seal to seal-damaging temperatures. Heat transfer gas is circulated to and from the second compartment by heat transfer conduits that are hermetically sealed from the vacuum chamber and the first compartment. An exterior magnet is positioned to induce a magnetic field in magnetic media disposed in the heat exchanger.
In another embodiment of the invention the vacuum chamber is formed of non-magnetic material and is adapted to receive and has disposed therein a heat exchanger formed of non-magnetic material. The heat exchanger includes first and second compartments. The first compartment is adapted to receive magnetic media and the second compartment has an airtight volume hermetically sealed from the first compartment and the vacuum chamber. A vacuum pump is connected to the vacuum chamber and is capable of providing a vacuum of at least 10xe2x88x927 Torr within the chamber. The heat transfer gas unit has heat transfer conduits connected to the second compartment of the heat exchanger to form a heat transfer gas circuit hermetically sealed from the vacuum chamber and the first compartment. The conduits supply heating and cooling gas to the second compartment. The first compartment is in thermal communication with the second compartment allowing conductive heat transfer between the gas supplied to the second compartment, the first compartment, and the magnetic media disposed therein. A magnet for inducing a magnetic field of greater than 0.25 Tesla in magnetic media disposed in the heat exchanger is located exterior to the chamber and positioned to induce the magnetic field along the center line of magnetic media disposed in the heat exchanger.
In another embodiment of the invention, the heat exchanger of the oven is located within, and is an integral part of the chamber, is spaced rearwardly of the door, and is adapted for hermetically sealed connection to the heat transfer gas unit external to the chamber.
In another embodiment of the invention, the heat exchanger of the oven is adapted to receive a carrier into which magnetic media are loaded.
The invention is further directed to a method of annealing magnetic media. The method comprises providing an oven according to the invention, loading magnetic media into the heat exchanger of the oven, closing the oven door to create a vacuum seal, ramping up to a vacuum of at least 10xe2x88x927 Torr within the chamber, ramping the temperature of the magnetic media to a target annealing temperature at a rate consistent with uniform heat transfer to the magnetic media, ramping up the magnetic field so as to provide a magnetic field of greater than 0.25 Tesla in the magnetic media disposed in the heat exchanger, holding the magnetic media at the target temperature while maintaining the magnetic field and the vacuum in the chamber, allowing the magnetic material of the magnetic media to assume a desired orientation, cooling the magnetic media to preserve the orientation of the magnetic material, ramping down and removing the magnetic field, ramping down the vacuum to gradually reintroduce atmosphere within the chamber; and unloading the magnetic media.